Chapter 18 — Key Takeaways

What you should leave Chapter 18 with

  1. A radical is a species with an unpaired electron. Radicals form by homolysis (each atom of a bond gets one electron).

  2. Homolysis vs heterolysis: heterolysis gives ions (Ch 10-17 chemistry); homolysis gives radicals. Single-barb (fish-hook) arrows show single-electron movement.

  3. Radical stability order (alkyl): 3° > 2° > 1° > methyl. Same trend as carbocations. Hyperconjugation explains it.

  4. Resonance-stabilized radicals: - Allyl ($CH_2=CH-CH_2^{\bullet}$): unpaired electron delocalizes via two resonance structures. - Benzyl ($PhCH_2^{\bullet}$): delocalizes into multiple resonance structures with the aromatic ring. These are dramatically more stable than typical alkyl.

  5. Bond Dissociation Energy (BDE): lower BDE = easier homolysis. - Methane C-H: 105 kcal/mol. - Benzyl C-H: 89 kcal/mol. - Allyl C-H: 88 kcal/mol. - Bis-allylic (in PUFAs): ~75 kcal/mol.

  6. Chain reactions have three stages: - Initiation: generate first radicals (peroxide thermolysis, X₂ photolysis). - Propagation: each step has a radical reactant and a radical product. Chain repeats many times. - Termination: two radicals combine to give non-radical; ends the chain.

  7. Chain length is typically 10⁴-10⁶: one initiation event leads to many product molecules. This is why radical reactions work efficiently with catalytic initiator.

  8. Radical halogenation (alkane + X₂ + light/heat): - Initiation: X-X homolysis. - Propagation: X• abstracts H (rate-limiting); R• takes X from X₂. - Selectivity: prefers H positions giving stable radicals (3° > 2° > 1°).

  9. Bromine vs chlorine selectivity: - Cl•: less selective (~5:1 for 3°:1°). - Br•: more selective (~1600:1 for 3°:1°). Hammond postulate: endothermic Br abstraction has late TS, reflects substrate stability.

  10. Anti-Markovnikov HBr addition (peroxide effect): radical chain mechanism. Br• adds to less-substituted C of alkene (radical at more-substituted C is stable). H abstraction from HBr completes. Net: Br at less-substituted C — anti-Markovnikov.

  11. Only HBr gives the peroxide effect cleanly. HCl (too strong H-Cl) and HI (too weak H-I) don't.

  12. NBS allylic bromination: NBS in CCl₄ + light brominates allylic C-H. Mechanism: NBS slowly releases low [Br₂]; Br• abstracts allylic H (resonance-stabilized radical); allylic radical + Br₂ → allylic bromide.

  13. NBS vs Br₂: Br₂ alone (high [Br₂]) adds across the C=C. NBS (low [Br₂]) brominates the allylic C-H selectively.

  14. Radical polymerization: alkene monomer + radical initiator → polymer chain. Used for:

    • LDPE (low-density polyethylene; branched, soft).
    • PVC (polyvinyl chloride; rigid).
    • PS (polystyrene; foam, packaging).
    • PMMA (Plexiglas).
    • PAN (acrylic fibers).
    • PVA (wood glue).

  15. Chain transfer in radical polymerization gives branching → LDPE's branched structure (vs. HDPE's linear from Ziegler-Natta).

  16. Combustion is a giant radical chain: alkane + O₂ + heat → CO₂ + H₂O. The exponential propagation explains why fires spread.

  17. Lipid peroxidation (biological radical damage):

    • Initiated by ROS (superoxide, hydroxyl radical) abstracting bis-allylic H from PUFAs.
    • Propagated by peroxyl radicals.
    • Damages cell membranes, proteins, DNA.

  18. Antioxidants quench radical chains:

    • Vitamin E (α-tocopherol): phenolic O-H donates to peroxyl radicals; resonance-stabilized phenoxyl results.
    • Vitamin C: water-soluble; recycles oxidized vitamin E.
    • Glutathione: thiol; broad antioxidant.
    • Carotenoids, polyphenols: dietary antioxidants.

  19. Modern photoredox catalysis (Ch 40): light + photocatalyst (Ru, Ir, organic dyes) generates radicals at room T with control. Used in modern synthesis and asymmetric reactions.

  20. Mastery of Chapter 18 explains:

    • Anti-Markovnikov HBr (Ch 15 set up the question; Ch 18 answers it).
    • NBS allylic bromination (used in synthesis).
    • Industrial polymer production.
    • Biological oxidative damage and antioxidant defense.

Cross-references

  • Chapter 15 — Markovnikov vs anti-Markovnikov; Ch 18 explains the radical mechanism for anti-Markovnikov HBr.
  • Chapter 16 — Alkene addition reagents (mostly ionic; Ch 18 is the radical alternative).
  • Chapter 17 — Alkynes (similar chain mechanism for some reactions).
  • Chapter 19 — Diels-Alder (concerted, not radical).
  • Chapter 27 — α-Carbon chemistry (ionic enolate; not radical).
  • Chapter 34 — Lipid biosynthesis; PUFAs vulnerable to peroxidation.
  • Chapter 36 — Reduction methods; NaBH₄ is hydride, not radical.
  • Chapter 40 — Modern photoredox catalysis.
  • Appendix B — pKa table (for comparing radical stability).
  • Appendix C — Reaction summary.

Study tip

For each radical problem, ask: 1. Where is the unpaired electron? Identify the radical. 2. Is it stabilized? Check for hyperconjugation (alkyl) or resonance (allyl, benzyl). 3. What's the chain step? Initiation, propagation (which step), or termination. 4. What's the selectivity? For halogenation: Br more selective than Cl; for HBr: ionic (Markovnikov) vs radical (anti-Markovnikov).

If you can answer these for any radical problem, you've internalized Chapter 18.